Technical Field
Embodiments of the subject matter disclosed herein generally relate to the field of hydrocarbon prospecting and extraction. In particular, the embodiments disclosed herein relate to apparatuses, methods, and systems for measuring the geo-mechanical properties of rock and adjusting hydrocarbon recovery operations in response to those measurements.
Discussion of the Background
Geophysical data is useful for a variety of applications such as weather and climate forecasting, environmental monitoring, agriculture, mining, and seismology. As the economic benefits of such data have been proven, and additional applications for geophysical data have been discovered and developed, the demand for localized, high-resolution, and cost-effective geophysical data has greatly increased. This trend is expected to continue.
For example, seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (either on land or seabed) that facilitates finding and extracting hydrocarbon reserves. While this profile does not provide an exact location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs.
Traditionally, a land seismic survey system 10 capable of providing a high-resolution image of the subsurface of the earth is generally configured as illustrated in FIG. 1 (although many other configurations are used). System 10 includes plural receivers 12 and acquisition units 12a positioned over an area 13 of a subsurface to be explored and in contact with the surface 14 of the ground. A number of seismic sources 16 are also placed on surface 14 in an area 17, in a vicinity of area 13 of receivers 12. A recording device 18 is connected to a plurality of receivers 12 and placed, for example, in a station-truck 20. Each source 16 may be composed of a variable number of vibrators or explosive devices, and may include a local controller 22. A central controller 24 may be present to coordinate the shooting times of the sources 16. A positioning system 26 (e.g. GPS, GLONASS, Galileo, and Beidou) may be used to time-correlate sources 16 and receivers 12 and/or acquisition units 12a. 
With this configuration, the sources 16 are controlled to generate seismic waves, and the receivers 12 record the waves reflected by the subsurface. The receivers 12 and acquisition units 12a may be connected to each other and the recording devices with cables 30. Alternatively, the receivers 12 and acquisition units 12a can be paired as autonomous nodes that do not need the cables 30.
The purpose of seismic imaging is to generate high-resolution images of the subsurface from acoustic reflection measurements made by the receivers 12. Conventionally, as shown in FIG. 1, the plurality of seismic sources and receivers is distributed on the ground surface at a distance from each other. The sources 16 are activated to produce seismic waves that travel through the subsoil. These seismic waves undergo deviations as they propagate. They are refracted, reflected, and diffracted at the geological interfaces of the subsoil. Certain waves that have traveled through the subsoil are detected by the seismic receivers 12 and are recorded as a function of time in the form of signals (called traces).
Once a promising region for hydrocarbon reserves is found, vertical and horizontal wells may be drilled to potentially extract the reserves. For example, in the United States and other regions of the world, there are many areas where oil shale rock deposits are to be found. Oil shale is a form of sedimentary deposits that were laid down eons ago, typically in the form of calcium carbonates, sodium carbonates, calcium bicarbonates, and quartz. Furthermore, soil materials and other compounds may have been entrapped in the matrix of the aforementioned materials.
While many oil shale reserves exist, most of them are located as deep deposits five to ten thousand feet below the surface of the earth. Since the early 20th century, many attempts have been made to mine or extract the oil from stratified shale formations. Although historically the shale oil proved to be a very suitable hydrocarbon product, the complexity of extracting oil shale reserves increased the cost of production well beyond the market price of similar products. Consequently, sustained shale production proved to be uneconomical.
Recently, the rapid development and exploitation of two specialized technologies has dramatically changed the cost of extracting oil from shale rock deposits. The first improvement is the carefully controlled and steerable directional drilling techniques that enable vertical drilling to be redirected into horizontal drilling at a selected depth. The drilling can then continue horizontally in a shale formation for a considerable distance.
The second improvement was the development of hydraulic fracturing techniques where slurry is pumped into a well at regular perforation points along an inserted casing in order to extend the economic life of the depleting oil fields. Although first used in the late 1940's, hydraulic fracturing has recently become a common technique to enhance the production of low-permeability formations, especially unconventional reservoirs—primarily tight sands, coal beds, and deep shales.
Despite many improvements, the cost of fracturing is still relatively high and significant inefficiencies remain. For example, many oil shale formations cross tectonic fault lines in the crust of the earth and thus can be discontinuous in their configuration. Some oil shale formations are slightly inclined in both the vertical and horizontal planes. Consequently, the abundance of oil may vary significantly as a function of drilling distance. In fact, it is estimated that approximately 30 percent of the perforation points in a typical fracturing operation correspond to dry regions where oil is unavailable.
Referring to FIG. 2a, in horizontal shale gas wells, fracturing is typically done in multiple stages at regular fixed intervals starting at the “toe” of the well (the name given to the tip of the foot-shaped horizontal wellbore) and proceeding toward the “heel” (the end of the horizontal section of the wellbore that is closest to the vertical portion). For example, a wellbore that extends 5,000 feet laterally within a shale layer might be hydraulically fractured in ten to fifteen stages several hundred feet apart. Typically, each perforation interval is isolated in sequence so that only a single section of the well is hydraulically fractured at a given time and to prevent damage to other sections of the wellbore.
During a hydraulic fracturing operation, fracturing fluid is pumped at high pressure through perforations in the section of the casing. The chemical composition of the fracturing fluid, as well as the rate and pressure at which it is pumped into the shale formation, are tailored to the specific properties of each shale and, to some extent, each well. When the pressure increases to a sufficient level, a planar hydraulic fracture opens in the rock, propagating more or less perpendicularly to the path of the wellbore. Although the fractures depicted in FIG. 2a are by necessity shown to be substantially vertical, the casing perforations in a well are typically oriented to produce fractures that propagate horizontally rather than vertically.
It should be noted that the fracturing characteristics of shale rock may vary significantly between wells or even within the same well. For example, soft oil shale formations respond differently than hard oil shale formations when subjected to the same level of hydraulic water pressure and soaking time. In addition to the mineral composition, the fracturing characteristics of shale formations may be dependent on the texture or fabric of the shale rock. For example, shale formations with larger and/or more abundant pores may fracture more easily than shale formations with smaller and less abundant pores.
Referring to FIG. 2b, a typical hydro-fracture may propagate horizontally about 500-800 feet away from the well in each direction. The fracturing pressure is carefully controlled to prevent vertical propagation beyond the thickness of the layer of gas-producing shale. The pressure needed to propagate the hydraulic fracture varies and depends on depth, the pressure of the gas in the pores of the shale, and the geo-mechanical properties of the hydrocarbon bearing layer, such as porosity. Consequently, reliable and readily available measurements of the geo-mechanical properties of the hydrocarbon bearing layer, as well as adjacent layers, can improve the effectiveness of hydrocarbon recovery operations.
A variety of techniques exist for measuring the geo-mechanical properties of rock retrieved from a well. Many of these techniques are indirect in nature and are typically limited to vertical wells. For example, a variety of physical property measurements that typically correlate with porosity have been used to predict the porosity of a sample or region. Examples include resistivity, sound velocity, gamma-ray back-scattering rates, chargeability (e.g., image logs) and neutron density. X-ray diffraction data may also be used to determine physical properties related to porosity. While such techniques are often useful they do not provide a direct measurement of the desired geo-mechanical property and often require the insertion of equipment into the well. However, with horizontal wells the pressures and temperatures involved and the inaccessibility to horizontal sections of the well typically prohibit the insertion of such equipment. Furthermore, making precise measurements of the geo-mechanical properties of rock (e.g., porosity and brittleness) with many of these techniques requires continued collection of core samples at regular intervals during the drilling process. Unfortunately, core samples currently cannot be effectively extracted from horizontal sections of wells where the geo-mechanical data is most needed.
When core samples are collected, the samples are sent to a laboratory for analysis. In addition to slowing the drilling process to remove the core samples, shipping a large number of core samples to a laboratory may be a costly and time consuming process. For example, getting results from a laboratory and analyzing those results may take weeks or even months before critical decisions about hydrocarbon recovery operations can be made.
Electron microscopy is used in many R&D facilities such as universities, commercial laboratories, and medical research laboratories as a technique for image and spectroscopic analysis. Improvements in hardware and software have allowed these instruments to become more widely used in various industries such as mining and semiconductor manufacturing. Although the increased flexibility, power, and robustness of these systems has enabled their use in a broader range of industries, the ability to gather and process the required quantity of statistically viable data for elemental, mineralogical and textural analysis has been a challenge. Efforts to resolve this challenge have seen the development of hardware and software to handle the spectroscopic aspect of SEM images resulting in wide use in the mining industry.
In contrast to spectroscopy, the use of high resolution back-scatter images of geological samples generated by electron microscopy has been limited to isolating the sample from the suspension media or for the purpose of energy dispersive spectrometry of the sample material. However, data regarding the pore spaces of rock has been neglected in those efforts. This lack of pore-space data has been further highlighted with recent advances in horizontal drilling that make the use of many traditional measures of porosity difficult if not impossible.
Given the foregoing, the ability to timely and accurately determine pore-space metrics for geo-physical samples without inserting measurement equipment into a well and without requiring core samples would advance the art of hydrocarbon exploration and extraction. For example, such metrics could be used to verify the presence of oil shale and determine the geo-mechanical properties thereof as a function of drilling distance. Furthermore, perforations in a well casing could be placed at locations according to the geo-mechanical properties of the surrounding rock resulting in a significant reduction in dry fracturing regions and a corresponding reduction in the cost of recovering hydrocarbon reserves from shale formations and the like.